Sinusitis is an inflammation of the sinuses (the paired air pockets within the bones of the face). Sinusitis is most often due to an infection within these spaces.

The sinuses, which are connected to the nose, are lined with the same kind of skin found elsewhere within the respiratory tract. This skin has tiny little hairs projecting from it called cilia. As the cilia beat constantly, they help move the mucus produced in the sinuses into the respiratory tract. As the mucus is swept along the respiratory tract, they help clear the respiratory tract of any debris or organisms.

When the lining of the sinuses is swollen, the mucus can't flow normally. Trapped mucus then fills the sinuses, causing an uncomfortable sensation of pressure and providing an excellent environment for the growth of infection-causing bacteria.

Sinusitis is almost always due to an infection, although swelling from allergies can...